In the field of data communications, communications networks typically utilize techniques designed to maintain or improve the integrity of signals being transmitted via the network (“transmission signals”). To protect signal integrity, the communications networks should, at a minimum, satisfy compliance standards that are established by standards committees, such as the International Organization for Standardization (ISO), International Electrotechnical Commission (IEC), or the Telecommunication Industry Association (TIA). The compliance standards help network designers provide communications networks that achieve at least minimum levels of signal integrity as well as some standard of compatibility.
One prevalent type of communication system uses twisted pairs of wires or other conduits to transmit signals. In twisted pair systems, information such as video, audio, and data are transmitted in the form of balanced signals over a pair of conduits, such as wires. The transmitted signal is defined by the voltage difference between the conduits.
Crosstalk can negatively affect signal integrity in twisted pair systems. Crosstalk is unbalanced noise caused by capacitive and/or inductive coupling between conduits of a twisted pair system. Crosstalk can include differential mode and common mode crosstalk, referring to noise created by either differential mode or common mode signals radiating from a transmission conduit. The effects of crosstalk become more difficult to address with increased signal frequency ranges.
Twisting pairs of wires together, such as in twisted pair systems, provides a canceling effect of the differential mode crosstalk created by each individual wire, as the effect of crosstalk created by one wire is compensated for by the corresponding voltage of the complementary wire.
Communications networks include connectors that bring untwisted transmission signals in close proximity to one another. For example, the contacts of traditional connectors (e.g. jacks and plugs) used to provide interconnections in twisted pair telecommunications systems are particularly susceptible to crosstalk interference. This is due in part to the fact that twisted pair wires are typically straight within at least a portion of the connector. Over this untwisted length, a complementary wire no longer provides compensation for wire-to-wire crosstalk. These effects of crosstalk increase when transmission signals are positioned close to one another. Consequently, communications networks connection areas are especially susceptible to crosstalk because of the proximity of the transmission signals.
Crosstalk can be described as a transmission line effect of a “disturbing wire” affecting a “disturbed wire”. In the case of cabling-to-cabling effects, the effects can be considered to be a “disturbing channel” on a “disturbed channel”. Crosstalk at a given point on a transmission line can be measured according to a number of components based on its source. Near end crosstalk (NEXT) refers to crosstalk that is propagated in the disturbed channel in the direction opposite to the direction of propagation of a signal in the disturbing channel, and is a result of the vector difference between the currents generated by inductive and capacitive coupling effects between transmission lines. Far end crosstalk (FEXT) refers to crosstalk that is propagated in a disturbed channel in the same direction as the propagation of a signal in the disturbing channel, and is a result of the vector sum of the currents generated by inductive and capacitive coupling effects between transmission lines.
An additional form of crosstalk, alien crosstalk, refers to crosstalk that occurs between different cabling (i.e. different channels) in a bundle or otherwise in close proximity, rather than between individual wires or circuits within a single cable. Alien crosstalk can include both alien near end crosstalk (ANEXT) and alien far end crosstalk (AFEXT). Alien crosstalk can be introduced, for example, at a multiple connector interface. This component of crosstalk typically has not presented a performance issue due to the data transmission speeds and encoding involved in existing systems.
Further, common mode signals can affect crosstalk between wires or wire pairs in a single cable or between cables in cabling. These common mode signals can have a detrimental effect upon performance because they can result in differential crosstalk at connectors within a network, adding to the crosstalk noise produced. At current network data transmission speeds, common mode signals have not produced a sufficiently detrimental effect for their consideration to be mandated in current standards.
In twisted pair systems various data transmission protocols exist, each having specific timing and interference requirements. For example, category 3 cabling uses frequencies of up to 10 MHz, and is used in 10BASE-T networks. Category 5 cabling, which is commonly used in 100BASE-TX networks operating at 100 Mbit/sec, operates at a frequency of up to 100 MHz. Category 5e cabling can be used in 1000BASE-T networks, and also operates at up to 100 MHz. Category 6 cabling, because of additional throughput needed, is specified to operate at 250 MHz. Category 6a cabling is currently specified to operate at frequencies of up to 500 MHz.
Many connectors use capacitive elements to compensate for the crosstalk between pairs in a plug and jack connector. Capacitive coupling can be used to achieve a compensative effect on either overall NEXT or FEXT, while having a detrimental effect on the other due to the additive/differential vector effect of each. With increasing data transmission speeds, additional crosstalk of various types is generated among cables, and must be accounted for in designing systems in which compensation for the crosstalk is applied.